Error message "FormatException: Input string was not in the correct format System.Int32.Parse (System.String s)..."

         if (GUI.Button (new Rect (Screen.width / 12.0f, Screen.height / 2.5f, 120, 40), "Volver")) {
             Application.LoadLevel("Main");
         }

The code also works but this error message appears and I don't know why...:Error message "FormatException: Input string was not in the correct format System.Int32.Parse (System.String s) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System/Int32.cs:629) Buy.Update () (at Assets/Scripts/Store/Buy.cs:39)"

(Buy is the name of the script which I've written this code)